The present invention relates to an apparatus for the in-circuit detection of defective capacitors of the type used on electric power distribution systems.
The shunt connected high voltage distribution type capacitor units are used extensively by the electric utility industry and manufacturing industry in their electrical supply systems for correcting the power factor of the system and maintaining voltage levels. A common method of construction of these capacitors consists of two sheets of pure aluminum foil separated by layers of kraft paper or plastic film impregnated with chromated aromatic hydrocarbon compounds. A capacitor unit is made up of several of these individual small sized capacitor packs or rolls which are connected in a parallel-series arrangement in order to obtain the desired capacitance value and withstand the high voltages in the range of 2000 volts to 20,000 volts. Before the capacitor unit is enclosed in a sealed metal can, having two insulated bushing-type terminals extending from the top, the metal can is filled with an insulating flammable liquid. Before 1977, most capacitor units contained PCB-type insulating fluids. However, such materials have been condemned because of toxicity and environmental problems. These capacitor units are connected to the distribution line at various locations close to the load centers and are installed singly or in parallel banks of two, three or four per line phase. Generally, only one fuse is employed to clear a bank of 2, 3 or 4 capacitor units, should an electrical short to ground occur on one of the capacitor unit cans.
It is important that as many of the capacitor units as possible in an electric distribution system be in a serviceable condition so that during periods of high electrical load the system voltage is held within an acceptable range. However, because of periodic failures and bank fusing, at any given time there may be a large number of capacitor units disconnected from the system and awaiting service. In order to compensate for these failed capacitor units, either a surplus of capacitor units over and above the minimum necessary is required, or the generators must supply the additional reactive power at the expense of producing real power. In either case, the result is an unacceptable additional monetary expense.
Prior to the present invention, detection of failed capacitor units by electric utility linemen has been primarily that of a visual observation. A blown fuse is one signal to a lineman that a bank of capacitor units is out of service. One or more of the capacitor units may be defective, or possibly the fuse was blown for a reason other than a shorted capacitor. If the lineman cannot visually detect a sign of failure, such as a bulged metal can or a leaking seam or bushing, the operating practice has been to re-energize the bank of capacitors with a new fuse.
The above practice has very serious drawbacks and limitations. First, should the capacitor bank be re-energized when a capacitor unit actually has an electrical short, for which no visual indication is apparent, there is the increased possibility of a violent rupture and release of the dielectric fluid onto the immediate area. Containment of the liquid dielectric fluid within the capacitor can is very important because the capacitor units containing PCB-type insulating fluids must not be allowed to contaminate the immediate area.
Secondly, a capacitor unit may have a partial or incipient fault because one of the small series of connected packs, making up the total capacitor unit, has shorted or opened. The capacitor unit may withstand the applied distribution line voltage for a short time, perhaps hours or weeks, by a redistribution of the line voltage across the remaining operative series connected capacitor packs. However, arcing by the shorted capacitor pack can cause the dielectric fluid and insulation to decompose into hydrocarbon gases, bulging the capacitor can and creating a potentially explosive condition when the eventual electrical failure of the capacitor unit occurs.
There have been several suggested methods and apparatuses for determining the condition of a capacitor unit, however, these suggestions all require operations which are normally beyond the assigned duties, and technical capabilities of the average lineman. For example, the USA Standards for Shunt Power Capacitors 0551-1968 describes various field tests for these type capacitor units. One such test requires the application of a known voltage and frequency of undistorted wave shape to the capacitor unit. However, this test of capacitance measurement requires disconnecting each capacitor unit from the remainder of the bank and then individually measuring the current drawn by each unit. Such tests require a high degree of technical competence to interpret the resultant meter readings and the manpower cost associated with such tests are prohibitive. Moreover, the disconnecting and handling of the capacitor unit can cause damage to the capacitor unit which results in incipient faults.
Also, commercially available capacitance meters, which apply a low AC voltage to provide a direct capacitance reading of the capacitor are unreliable on paper film capacitors. The gases created by arcing in a defective unit insulates the defective pack and capacitance readings obtained may be in error. Use of this device requires also disconnecting each unit in the capacitor bank during testing, which results in large manpower costs and expensive time loss during testing.
Lastly, the methods of determining the condition of the capacitor by measuring the current through the capacitor by ordinary split-magnetic core type ammeters lack sensitivity and are insufficiently shielded from the ambient electric and magnetic fields of the adjacent electric distribution lines to provide the necessary accurate readings to determine the condition of a capacitor unit. Accordingly, the measurement of the charging current to a specified single capacitor connected in parallel with similar capacitors utilizing an ordinary clamp-on type ammeter results in inaccuracies, due to the surrounding electrical and magnetic fields, which are unacceptable for determining the good/no good condition of that capacitor.